In recent years, small thin image pickup devices have been incorporated in a mobile terminal device which is a small thin electronic device such as a mobile telephone, a PDA (Personal Digital Assistant) and the like, whereby it becomes possible to transmit not only voice information but image information to a remote place mutually among the mobile terminal devices.
As an image pickup element used for these image pickup devices, solid state image pickup elements, such as a CCD (Charge Coupled Device) type image sensor and a CMOS (Complementary Metal-Oxide Semiconductor) type image sensor, are employed. In recent years, the miniaturization of the pixel pitch of an image pickup element has progressed, and high resolution and high performance have been attained by high pixel density. On the other hand, the miniaturization of the image sensor is attained while a high pixel density is maintained.
Moreover, as a lens to form an image of an object on these image pickup elements, a lens made of resin suitable for mass production has been employed in order to attain more low cost. Since such a lens made of resin has a good processability, a technique to form an aspheric surface on the lens has responded to a request for high performance. As such a lens, a lens structure constituted with two lenses is proposed by Patent Documents 1 to 9.
Patent Document 1: U.S. Pat. No. 7,492,528
Patent Document 2: Japanese Unexamined Patent Publication No. 2007-148407
Patent Document 3: Japanese Unexamined Patent Publication No. 2009-42333
Patent Document 4: Japanese Unexamined Patent Publication No. 2006-178026
Patent Document 5: Japanese Unexamined Patent Publication No. 2004-170460
Patent Document 6: Japanese Patent Publication No. 4064434
Patent Document 7: Japanese Unexamined Patent Publication No. 2008-292651
Patent Document 8: Japanese Unexamined Patent Publication No. 2009-103896
Patent Document 9: Japanese Unexamined Patent Publication No. 2009-103897
However, in the image pickup lenses disclosed by the Patent Documents 1 and 2, all lenses are constituted with positive lenses, and astigmatic is not corrected sufficiently so that the image pickup lenses cannot respond sufficiently to the high pixel density.
On the other hand, Patent Documents 3 through 9 disclose an image pickup lens in which the second lens is made as a negative lens. However, there is a problem that curvature of image field is large or chromatic aberration of magnification is not corrected sufficiently.
Moreover, in recent years, as a method of mounting a lens module on a base boar at low cost in large quantity, proposed is a method in which a lens module is installed together with an IC (Integrated Circuit) chip and other electronic components on a base board on which solder is potted beforehand and is subjected to a reflow process (heating process) to melt the solder so that the lens module and the electronic components are mounted simultaneously on the base board. Therefore, there is a real situation that an image pickup lens excellent in heat resistance to be endurable for the reflow process is required.
The present invention has been made in view of such a situation, and an object of the invention is to provide an image pickup lens, an image pickup device employing the image pickup lens and a mobile terminal device in which chromatic aberration can be corrected up to a periphery of an image plane with a small number of lenses and the image pickup lens has a high performance, is small and corresponds to an image pickup element with a high pixel density.
Here, With regard to the criterion of a small image pickup lens, the present invention aims the miniaturization of the level to satisfy the conditional formula (12). When the range is satisfied, the overall length of a lens can be shortened and the outer diameter of the lens can be made also small synergistically. With this, it becomes possible to make the entire body of an image pickup device small and light.TL/2Y<1.5  (12)
where TL represents a distance on an optical axis from a lens surface located closest to an object in the entire system of the image pickup lens to an image side focal point, and 2Y represents a length of a diagonal line of an image taking surface of a solid state image pickup element (a length of a diagonal line of a rectangle effective pixel region of a solid state image pickup element).
Herein, “image side focal point” means an image point when light rays in a finite object distance described in Tables 1 to 13 in Examples enter. Further, in the case that a parallel plate such as an optical low pass filter, an infrared ray cut filter, a band pass filter, and a seal glass of a solid state image pickup element package is located between a lens surface position closest to an image side in an image pickup lens and an image side focal point, the parallel plate portion is made to an air conversion distance and the above TL is calculated by the use of the air conversion distance.
Furthermore, it is more desirable to satisfy the conditional formula (12′). When the range specified by the conditional formula (12′) is satisfied, a lens can be made more small and light. In addition, since an effective radius in which actual light rays pass through becomes small, it becomes possible to make a space to arrange a mechanism for short distance photography around the lens. As a result, high performance can be attained while the size is maintained.TL/2Y<0.9  (12′)